Current limiting fuse



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UnitedStates Patent O 3,483,502 CURRENT LIMITING FUSE Harvey W. Mikulecky, Racine, Wis., assignor to McGraw- Edison Company, Milwaukee, Wis., a corporation of Delaware Filed Dec. 21, 1967, Ser. No. 692,357 Int. Cl. H01h 85/30 U.S. Cl. 337-244 11 Claims ABSTRACT OF THE DISCLOSURE A current limiting fuse enclosed in an insulating fuse tube and including a fusible element helically wound on a support core for interconnecting terminals at the opposite ends of the casing and a granular insulating material filling the casing. An indicator wire is helically wound on the support core in a general parallelism with a portion of the fusible element and is isolated therefrom and from the terminals. The indicator wire normally holds a spring-loaded indicator member and parts when subject to an arc at one of the terminals after the fusible element has parted to release the indicator member.

BACKGROUND OF THE INVENTION This invention relates to current limiting fuses and, more particuarly, to a new and improved indicator for such fuses.

One type of current limiting fuse generally includes one or more fusible elements which may be helically wound on a central core so as to provide a sufficient element length within a reasonably sized housing. In order to give a visual indication that the fuse has operated, an indicator may be provided which includes a spring operated plunger held by an indicator wire which parts when the fusible element melts. Such indicator wires are generally connected to one terminal of the fuse and are helically wound on the core for the full length of the fuse tube, except for a small gap intermediate its ends. Indicator wires which runs the full length of the fuse tube tend to provide an internal arc-over path through the full length of the fuse housing in the event of a failure.

In the operation of prior art current limiting fuses, the fusible elements first part in one or more locations, after which the gap in the indicator wire flashes over causing the current to be switched to the indicator wire which then vaporizes. Upon this event, the current is switched back to the segments of the fusible element which then vaporize causing a high voltage surge.

It is an object of the invention to provide an indicator for a current limiting fuse wherein the indicator wire does not provide an arc-over path through the full length of the fuse tube.

Another object of the invention is to provide a current limiting fuse wherein various portions of the fusible element vaporize at different times during the interruption of high current faults to reduce the magnitude f the resultant voltage surge.

These and other objects and advantages of the invention will `become apparent from the detailed description thereof taken with the description of the preferred embodiment set forth hereinbelow.

SUMMARY OF THE INVENTION A current limiting fuse including a fusible element for interconnecting spaced apart terminals and an indicator wire disposed in general parallelism with the fusible ele ment and extending for only a partial distance through the fuse tube and having one end disposed in a gapped relation to one terminal wherein the indicator Wire will part at the gap when the fuse operates to release an indicator member and to momentarily shunt the current from one portion of the fusible element.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows a fuse assembly incorporating an indicator according to the instant invention;

FIG. 2 schematically illustrates the fuse assembly shown in FIG. l;

FIG. 3 is an enlarged fragmentary view of the fuse assembly shown in FIG. 1;

FIG. 4 is a view taken along lines 4-4 of FIG. 3; and

FIG. 5 is an enlarged View showing the indicator member of the fuse assembly shown in FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 shows a current limiting fuse 10 incorporating the instant invention and including a tubular casing 11 formed of a suitable insulating material such as glass, fiber, or glass impregnated with epoxy resin. The opposite ends of tube 11 are closed by metallic conductive terminal members 12 and 13 which may be aiiixed in a sealing relation in any suitable manner, such as by epoxy cement. Those skilled in the art will appreciate that the terminals 12 and 13 may include means (not shown) for mechanically supporting the fuse 10 and for electrically connecting it into the circuit being protected.

The fuse 10 also includes one or more fusible elements 14 which may take the form of thin wires of conductive material, such as silver or copper, and which melt when subject to a predetermined temperature in a manner well known in the art. The wires 14 are helically wound on an elongate insulating core member 15 which extends axially within the casing 11 and which is generally X-Shaped in cross-section and formed by a plurality of radially protruding, peripherally spaced apart, longitudinally extending iins 16.

Conductive, tubular end members 18 and 19 are affixed to each end of the core member 15, and each is in turn affixed to the terminal members 12 and 13, respectively, to join the members in a unitary assembly. Each end member 18 and 19 has a plurality of radially extending tabs 20 adjacent its outer periphery, and forming terminals to which the fusible wires 14 may be suitably joined by solder or the like.

The space within the casing 11 and around the core member 15 and the fusible conductors 14 is filled with a suitable pulverulent refractory arc quenching material such as quartz sand 21.

A fusible indicator wire 22 is helically wound on the core 15 in spaced relation between the helical turns of, and in general parallelism with, the fusible elements 14. As seen in FIGS. l and '2, the indicator wire 22 is approximately one-half the length of the fusible elements 14 and extends substantially one-half the length of the core 15. As seen more clearly in FIGS. 3 and 4, one end of the indicator wire 22 may be anchored by means of an eyelet 24 disposed in an aperture 25 formed in one of the fins 16. In order to maintain the wire 22 in a taut condition, it is passed around a notch 27 cut in an adjacent iins 16 and located beyond the eyelet 24 and extends backwardly to eyelet 24 where it may be aiiixed in any suitable manner, such as solder 28.

In the preferred embodiment of the invention seen in FIG. 4, the aperture 25 may be located inwardly of the outer edge of the iin 16 so that the fusible elements 14 pass above the eyelet 24. As a result, there is no direct electrical connection between the fusible elements 14 and the indicator wire 22 at this point, as schematically illustrated in FIG. 2.

Referring now to FIGS. 1 and 5, the terminal member 12 is shown to have a central bore 30 for receiving the end member 18 which, in turn, encloses a nonconductive indicating plunger 32 that is urged in an axially outward direction by a spring 33 but normally restrained against a nonconductive washer 35 'by the indicator wire 22. The nonconductive washer 35 is supported by a conductive washer 36 which is suitably secured to a shoulder 38 formed on the internal surface of sleeve 18 so that washer 36 is in circuit with the fusible elements 14.

The indicator wire 22 is anchored on a notch 39 formed in one iin 16 of core 15 and passes through central apertures 40 and 41 in washers 35 and 36, respectively, and an oblique hole 42 formed in a disc portion 44 formed integrally on the core member and upon which the sleeve 18 is secured. The aperture 40 is substantially smaller than the aperture 41 and serves to centrally locate the wire 22 in spaced relation frOm the margins of the aperture 40. The terminal end of wire 22 is secured in any suitable manner, such as by solder 45, to an eyelet 46 which passes through an aperture 47 in the indicator plunger 32 and is secured in an insulating masking washer 48 which normally covers the outer end of plunger 32.

As will be more fully explained hereinbelow, when the fuse 10 is subject to -a predetermined current, fusible elements 14 will part, and this, in turn, will cause the indicator wire 22 to part so as to release the indicating plunger 32 for movement outwardly of the sleeve 18 under the inuence of the spring 33, and until it engages a stop ring 50 secured to the outer end of sleeve 18, as shown by phantom lines in FIG. 5.

The indicator plunger 32 may be of some bright color which is normally masked by the washer 48. However, when the indicator wire 22 parts, the eyelet 46 will be released from the aperture 47 in the plunger 32 so that the masking washer 48 will fall away to uncover'the plunger 32.

As seen in FIGS. 1 and 2, the indicator wire 22 extends for only approximately one-half the length of the fusible element 14 and, in addition, is preferably isolated therefrom by the gap between the eyelet 24 and the fusible elements 14 and from the terminal 11 by the gap that exists between the indicator wire 22 and the periphery of the aperture 41 in the washer 36. Initially, therefore, no current flows through the indicator wire 22 so that it can in no way affect the time current characteristic of the fuse 10. This is true even though there may be contact between one end of the wire 22 and the fusible element 14 and to the eyelet 24.

The fusible elements 14 may be provided with beads 51 which ane composed of a low melting temperature alloy, such as tin-lead solder, and which are in intimate Contact with the fusible elements 14 and preferably at the mld-point thereof. At the time current operating point of the fuse, elements 14 become hot enough to melt the bea'ds 51 and the amalgamation of the silver and the bead alloy causes a hot spot with a high enough resistance to melt the elements 14 at this point.

In the absence of the indicator wire 22, initial separation of the fusible elements 14 will cause these elements to separate into a plurality of segments interconnected by arcs and which substantially simultaneously vaporized. The vapor thus formed is disbursed among and condenses on the relatively cool granules of the surrounding arc quenching material 21 and is thereby no longer available for current conduction as the arc energy is dissipated in this heat transfer. This has the effect of inserting a relatively large resistance in the circuit to sharply limit the current through the fuse to some value less than the prospective fault current and gives rise to a voltage surge which is initiated at approximately the instant of vaporization and which is potentially damaging to the System.

The indicator wire 22 operates to limit the surge voltage incident to a current interruption by delaying the vaporization of a portion of the fusible elements 14. More specifically, as the current interruption is initiated, the indicator wire 22 provides a low resistance shunt path across approximately one-half of the fusible elements 14 and is, accordingly, switched into a parallel circuit relation by arc-overs at the eyelet 24 and the aperture 41 in the conductive washer 36. This arcing separates the indicator wire at the aperture 41 to provide the indication as described hereinabove and, in addition, momentarily shunts a portion of the elments 14. During this time, the remainder of the elements 14 vaporized to cause a voltage surge whose peak amplitude is lower than that which would normally occur if the entire elements 1 4 vaporized simultaneously. When the fault current is switched to the indicator wire 22, it also vaporizes and the current is switched back to the remaining portion of the fusible elements 14, and these, in turn, vaporize to produce a second voltage surge which is also of reduced magnitude. In this manner, the indicator wire 22 delays the vaporization of approximately one-half of the fusible elements 14 to provide two successive voltage surges, each of which is of substantially lower magnitude than would normally occur.

Current limiting fuses of this type sometimes fail as the result of an internal ilashover along the length of the core 15 and between adjacent turns of the fusible elements 14. The presence of an indicator wire extending the full length of the fuse 10 enhances the possibility of such failures. However, by limiting the length of the indicator wire to substantially one-half the length of the fusible elements, this possibility of internal flashover is substantially reduced.

While only a single embodiment of the invention has been shown and described, and while the invention has been discussed with respect to a pair of relatively thin fusible wires 14, those skilled in the art will appreciate that the invention also contemplates the use of a greater or lesser number of fusible elements, or the use of a relatively wider ribbon of fusible material. In addition, while only a single embodiment of the invention has been shown and described and while the invention has been described with respect to a particular type of current limiting fuse, it is not intended to be limited thereby, but only by the scope of the appended claims.

I claim:

1. A current limiting fuse comprising a tubular insulating casing, conductive terminal means mounted on each end of said casing, at least one elongate fusible element extending through said casing and interconnecting said terminals, and indicator means mounted on one of said terminals, indicator wire means restraining the operation of said indicator means and extending for substantially one-half the distance through said casing in general parallelism with said fusible element, one end Iof said indicator wire being electrically insulated from said one terminal and passing in close proximity thereto, the other end of said indicator wire terminating intermediate the ends of said casing.

Z. The'l current limiting fuse set forth fn claim 1 wherein an insulating core member extends generally parallel to the axis of said casing, said fusible element being generally helically wound on said core member and said indicator wire being wound on said core member in generally spaced relation with respect to said fusible element, and an inert granular material of high dielectric strength disposed in said casing and in surrounding relation to said core and fusible element and indicator wire.

3. The current limiting fuse set forth in claim 1 wherein said one end 0f said indicator wire is secured to said indicator means by an eyeletted insulating washer to restrain the operation thereof, and insulating gap means electrically connecting said one end of said indicator wire and said one terminal by an electrical arc during an overcurrent fault.

4. The current limiting fuse set forth in claim 2 wherein said other end of said indicator wire is anchored by an eyelet on said core member.

5. The current limiting fuse set forth in claim 2 wherein said indicator means includes an indicator member, resilient means urging said indicator member outwardly of said casing, and an insulating masking washer fastened to said indicator wire and normally covering and holding said indicator member against said resilient means, said indicator wire being severed by an electrical arc adjacent said one terminal after said fusible element parts, to release said insulating masking washer so that said indicator member may move outwardly of said casing.

6. The current limiting fuse set forth in claim 3 wherein said insulating gap means comprises a conductive disc member aixed to said terminal means and having an aperture formed therein, said indicator wire passing through said aperture, and nonconductive means attached to said conductive disc member for holding said indicator wire generally centrally of said aperture.

7. The current limiting fuse set forth in claim 3 wherein the other end of said indicator Wire is anchored by an eyelet that is electrically insulated from said fusible element and in close proximity thereto.

8. The current limiting fuse set forth in claim 3 wherein said indicator means includes an indicator member, resilient means urging said indicator member outwardly of said casing, and an insulated masking washer fastened to said indicator wire and normally covering and hold said indicator member against said resilient means, said indicator wire being severed by an electrical arc adjacent said one terminal after said fusible element parts to release said masking member so that said indicator member may move outwardly of said casing.

9. The current limiting fuse set forth in claim 2 wherein there is an insulating gap means comprising a conductive disc member aixed to said terminal means and having an aperture formed therein, said indicator wire passing through said aperture, and nonconductive means attached to said conductive disc member -for holding said indicator wire generally centrally of said aperture.

10. A current limiting fuse comprising a tubular insulating casing, conductive terminal means on each end of said casing, and insulating core within said casing, at least one `fuse element attached to said insulating core and interconnecting said terminals, indicator means mounted within the terminal on one end of said casing, indicator wire means restraining the operation of said indicator means and extending for substantially one-half the length and in general parallelism with said fuse element, insulating gap means for electrically isolating in close proximity said indicator wire means and said terminal on one end of said casing, and spring biasing means for forcing said indicator means outwardly from said terminal on one end of said casing when an electrical arc strikes from said indicator wire means to said terminal on one end of said casing during an overcurrent fault.

11. A current limiting fuse as defined in claim 10 in which said insulating gap means is formed between said indicator wire means and the inner diameter edge of an apertured metallic disc joined to said terminal on one end of said casing, said indicator wire means extending centrally through said apertured metallic disc and being held centrally through an apertured insulating disc positioned adjacent said apertured metallic disc, the aperture in said metallic disc being larger than the aperture in said insulating disc, thereby constructing a precise insulating gap Afor arc severing purposes.

References Cited UNITED STATES PATENTS 1,081,213 12/1913 Conant 337-244 2,523,139 9/1950 Owens 337-241 3,243,552 3/1966 Mikulecky 337-162 BERNARD A. GILHEANY, Primary Examiner H. B. GILSON, Assistant Examiner U.S. Cl. X.R. 337-159 

